Impedance measurement of grounding systems with alternative fall-of-potential method

The classical fall-of-potential method (potential probe is between grounding grid and current electrode) is universally used for the measurement of grounding impedance. However, in an urbanized area very often the potential probe cannot be located at the proper position, because of the presence of buildings, paved lots, underground metallic pipes, etc. In those cases the direction of the potential probe needs to be placed under angle α=90°-270° towards the direction of grounding grid current electrode, a method known as te alternative fall-of-potential method. The measured impedance, which in the case of small and medium sized grids is pure resistance, is always smaller than the true impedance. The purpose of this paper is to calculate errors of grounding resistance for different types of grids buried in two-layer soils, when the potential probe is under some angle to the straight line grounding grid-current electrode. The obtained results of the errors can be shown as a family of curves, which can be used in practice for correction of measured grounding grids resistance values